3.5 Shapes of Molecules

This topic explains how the Valence Shell Electron Pair Repulsion (VSEPR) Theory is used to determine the shapes and bond angles of molecules.

Definition

VSEPR Theory: Valence Shell Electron Pair Repulsion Theory — a model used to predict the shape of molecules based on the repulsion between electron pairs around the central atom.

Main Idea

Electrons are negatively charged, so electron pairs around a central atom repel each other. The shape of a molecule is determined by the arrangement that minimizes these repulsions. The shape depends on the number of bonding pairs and lone pairs around the central atom. Lone pairs occupy more space than bonding pairs, so they affect bond angles.

Core Concept

• Electron pairs around a central atom arrange themselves as far apart as possible to minimize repulsion.
• Lone pair-lone pair repulsion > lone pair-bond pair repulsion > bond pair-bond pair repulsion.
• The shape of the molecule is determined by the positions of the atoms only (not the lone pairs).
• The bond angle decreases as the number of lone pairs increases.

Detailed Explanation

Types of Electron Pairs

• Bonding pairs (BP): Electron pairs involved in covalent bonds.
• Lone pairs (LP): Electron pairs not involved in bonding.

Repulsion Order

LP-LP > LP-BP > BP-BP

Lone pairs repel more strongly than bonding pairs because they are held closer to the central atom and their charge cloud is more diffuse.

Shapes of Molecules Based on Electron Pairs